Excitation devices of this kind are used, for example, in the so-called high-induction testing of stators of large power station generators. For this type of testing, the stator is excited at mains frequency to a magnetic induction of approximately 1.5 Tesla.
The technical situation relating to high-induction stator measurement on power station generators in particular will be discussed here.
By this measuring process, the stator of a generator is magnetically excited by means of excitation coils and an alternating electrical current flowing through these coils. A conventional device is shown in FIG. 1. The figure shows the annular stator of a hydro generator in schematic form. The rotor is removed for this test. The windings of an excitation coil are arranged around this stator ring. For large stator diameters, individual coil segments are typically distributed symmetrically along the stator circumference for this purpose.
The excitation coil is operated with alternating voltage at mains frequency. With large generators, considerable voltages and currents and a large inductive reactive power are required in order to achieve the required nominal induction of 1.5 Tesla. Typical voltage values are several kV at a current strength of several kA, which means a reactive power requirement of several MVAr (energy content approx. 450 J/m3 and reactive power 2*50*450 Ws/s=45 kVAr/m3). This high reactive power requirement is one of the main problems when carrying out this measurement.
The electrical energy source (mains connection or generator) is often unable to supply this high unbalanced load. Furthermore, the high voltages used are basically hazardous to life, which means a significant outlay for safety measures. For these reasons, the measurement is very laborious. However, it is a compulsory requirement by many customers, especially in the acceptance testing of new generators after fabrication.
The reactive power requirement can be reduced by compensation by means of capacitors. However, there is then still the problem of high voltages, as the winding does not usually consist of one turn but has to be distributed around the circumference of the stator by means of a plurality of turns in order to effect a uniform field strength distribution.
A further solution consists in using an electronic power converter for supplying the excitation winding. The reactive power requirement of the excitation coil can be compensated by means of a suitably sized link circuit for the supply of energy. An advantage of this solution is that all load cases up to a maximum reactive power requirement can be covered. However, with this solution there is the problem that the converter has to be rated for the highest possible load case, which gives rise to relatively high costs. This rating relates in particular to the link circuit, which must absorb the reactive energy, and also to the electronic output components, which must withstand high voltages and currents.
Moreover, high voltages and currents which can endanger personnel are again present in the excitation coil.